The sensing of a pressure difference is important in the operations of many systems such as microphones, static pressure gauges and shear stress measuring devices. Flexible diaphragms in combination with various readout schemes have been used to detect pressure difference across the diaphragm. Pressure difference across a flexible diaphragm causes the diaphragm to deform. The readout scheme measures this deformation as a function of applied load and thereby provides a measurement of the sensed pressure difference.
Typical readout schemes involve a piezoresistive array in the diaphragm, or a movable plate capacitor associated with a fixed plate, or fiber optics. One disadvantage with electronic and capacitor schemes is that they are temperature sensitive and cannot be exposed to hostile environments.
In the case of shear stress measuring devices, measured pressure can be directly related to wall shear stress. Typically, pressure is transmitted from the area of a target wall to a remote location for determination of magnitude with respect to known pressures. However, the overall structure of flow over a wall comprises both a mean and fluctuating part of shear stress. The mean value determines the drag characteristics of a particular flow configuration, while the fluctuating part is of importance in sound generation, separated flows, passive or active control of turbulence and in general, assessment of which types of flow structures are primarily responsible for momentum transfer between the outer part of the boundary layer in turbulent flow and the wall. Further, non-lateral fluctuating forces, such as environmental pressures and eddies, affect the measuring of wall shear. It is known that many shear stress measuring devices which directly relate measured pressure to wall shear stress are not suitable for the measurements of fluctuating shear stress.
For example, a Stanton tube measures shear stress by employing a protruding member connected to one end of a tubing and a pressure transducer connected to the opposite end of the tubing remotely located from the target flow. The protruding member is positioned in the target flow in a manner that protrudes just above the wall on which shear stress is to be detected. An opening through the protruding member faces upstream into the target flow and enables fluid communication to the one end of the tubing. Pressures from the target flow are transmitted by the tubing from the opening of the protruding member to the pressure transducer. The pressure measurement produced by the pressure transducer is directly related to wall shear stress. However any fluctuations in pressure from the target flow are also transmitted by the tubing from the opening to the pressure transducer. Such fluctuations and any asymmetries in inner diameter of the tubing along the length of the tubing (e.g. at joints or connectors) cause a pumping force to be experienced by the pressure transducer. As a result, an accurate pressure measurement, and hence shear stress measurement, can not be obtained. Further the Stanton tube is not useable in a type of flow (laminar versus turbulent) for which it is not calibrated. That is, if there is a change in the nature of the boundary layer and the wall pressure fluctuations, then the Stanton tube will fail to provide dependable shear stress measurements.
In addition, the Stanton tube method of measuring shear stress can not discriminate between pressures that are uniform over a certain scale (size) and those that are uniquely related to shear stress. This is also true if the Stanton tube were modified by placing the pressure transducer at the opening though which flow generated pressure is detected.
Accordingly the measuring of wall shear stress, including fluctuating shear stress, is not a trivial matter.
As between uses of diaphragm pressure sensors, most such sensors are not easily transferred from use to use, are costly and often impractical.